Ignitron excitation control circuit



May 14, 1963 G. RANDOLPH 3,089,984

IGNITRON EXCITATION CONTROL CIRCUIT Filed Dec. 20, 1960 I 46 44 L i 24 0 so 90 I80 53 i l4 I2 I I6 IO A figzu Q B U 2232 34 36 52a V I 28 26 30 0 I 92b 42 L sc flg2c FIRING POTENTIAL FIG. I FIG. 2

INVENTOR. GERALD RANDOLPH ATTORNEYS I 3 089 984 IGNITRON EXCITATION CONTROL CIRCUIT Gerald Randolph, Franklin Park, N.J., assignor to Wallson Associates, Elizabeth, N.J., a corporation of New Jersey Filed Dec. 20, 1960, Ser. No. 77,163 4 Claims. (Cl. 315-168) This invention relates to a control circuit and, more particularly, relates to an improved ignitron excitation control circuit.

United States Patent Excitation control circuits for i-gnitrons have generally taken the form of anode firing circuits and separate excitation supply circuits.

The anode firing circuits, in which the excitation power for the igniter electrode is derived from the anode supply, requires operation of the control circuit at the potential of the supply source. Thus, the components are expensive. In addition, the anode firing excitation supply dissipates power derived from the supply source during the entire ignitron firing cycle, requiring a power transformer of higher power rating.

Although the separate excitation supply has the advantage *of operation at the lower igniter striking voltage level, the excitation supplies known to the art have not offered the requisite reliability of firing of the ignitron with a supply of the desired low cost and ease of construction. The high average power dissipated in such separate supplies has resulted in the requirement that relatively expensive components be employed. In addition, the separate excitation supplies known to the art have not provided the desired high peak current for reliable ignition with a low r.m.s. value of igniter current for ex- .tended ignition tube life.

It is, therefore, the primary object of this invention to provide an improved anode firing circuit overcoming the disadvantages of the prior art.

Other objects and advantages will be pointed out hereinafter.

The invention will be more clearly understood by reference to the following description taken in combination with the accompanying drawings, of which:

FIG. 1 is a schematic diagram of a circuit constructed in accordance with the present invention;

FIG. 2 is a plot of waveforms in which amplitude is n plotted along the scale of ordinates against time plotted along a common scale of abscissa in which FIG. 2a shows the waveform developed at point A in the circuitry shown in FIG. 1; FIG. 2b is a plot of the circuitry at point B of FIG. 1; and FIG. 2c is a plot of the waveform at point C of FIG. 1; and

FIG. 3 is a schematic diagram of another embodiment of the present invention.

In FIG. 1 there is shown an ignitron 10 having a plate 12, mercury pool cathode 14 and igniter electrodes 16. The ignitron is serially coupled with resistor 18, representing the utilizing load, across the secondary winding 20 of transformer 22. The primary winding 24 is coupled across an alternating source 25, the phase of which shall be called phase I for the purpose of explanation.

The ignitron 10 will, of course, operate in conventional tashion preventing current flow through the load resistor 18 until the ignitron is fired by striking an are between the mercury pool cathode and the igniter electrode. Control of the time of striking this are with respect to the phase of the excitation voltage of transformer 22 will control the rms value of the current flowing through the load resistor 18.

To strike the igniter are at a time which is controllable with respect to the phase of the source 25, there is provided a second transformer 26 having a primary winding ice 28 and a secondary winding 30. The primary winding 28 is coupled across a source of alternating voltage 29 having a leading phase characteristic with respect to the voltage of source 25.

The secondary winding 30 is coupled to the ignitercathode circuit of the ignitron 10 by the serially connected current limiting resistor 32 and controlled rectifier 34.

The controlled rectifier, such as a silicon controlled rectifier, prevents current flow through the igniter circuit of the ignitron until the rectifier is fired by application of a firing pulse ,to the gate electrode 36. To controllably apply the gating voltage to the gate electrode, there is provided a transformer 38 having a primary winding 40 and secondary winding 42. The gate-cathode circuit of rectifier 34 is coupled across the secondary winding 42 by the serially connected resistor 44 and rectifier 46. The excitation supply for the primary winding 40 is derived from source 25 through the variable tap 48 on autotransformer winding 50, which winding is coupled directly across source 25. Alternatively, a dropping resistor with a variable tap may be employed in place of autotransformer 50 at some increase in power dissipation.

The operation of the circuit may best be understood by reference to the schematic diagram of FIG. 1 in combination with the waveforms of FIG. 2.

The gating voltage applied across the plate-cathode elements of the controlled rectifier is a sine wave, leading with respect to the sine wave applied to the gate electrode thereof. The phase of source 29 may be conveniently selected as degrees leading with respect to source 25 since such leading phase is easily obtainable from conventional three-phase supplies. The peak amplitude of the sine wave applied to the gate electrode (FIG. 2c) is dependent upon the setting of tap 48. Since the gate firing potential remains fixed, amplitude control of the conduction angle of the controlled rectifier 34 is provided by the setting of tap 48.

Whenthe instantaneous amplitude of the gate potential reaches the firing potential, rectifier 34 will fire, the

instantaneous amplitude of the potential across secondary 30 will be applied to the igniter [circuit and a current having a steep front (FIG. 2b) will flow through the igniter circuit of ignitron 10 striking an arc between the igniter and cathodes. The waveform through the igniter electrode then follows a sinusoidal waveform corresponding to the waveform of the voltage waveform applied by transformer 36. Since the current through the igniter circuit is only a portion of a full sinusoidal wave, high peak currents may be obtained with a low rms value of current through the igniter circuit. Thus, reliable firing of the ignitron 10 may be efiected without exceeding the power dissipation rating on the igniter electrode of the ignitron.

The current through the igniter circuit will initiate an arc and the ignitron 10 will conduct passing current through the load circuit 18.

Thus, there is provided a simple but efiicient amplitude control of the phase of the ignitron. Variation of the tap setting 48 will shift the position of the wave front as indicated by arrow 52 in FIG. 2b, and thus shift the start of the conduction cycle of the ignitron as, for example, iliustrated by the dotted waveform 53 in FIG. 2a.

In addition to providing a high peak current with low rrns igniter current, the short rise time of the igniter current pulse tends to improve the reliability of firing of the ignitron. Since the control circuit discharges into the igniter circuit over only a small portion of the total ignitron conduction angle, the power dissipation is low and transformer 26 can have a relatively low power rating. Further, the remaining components are relatively cheap and small.

On the reverse cycle of the alternating voltage, the ignitron will extinguish, readying the circuit for the next openating cycle.

Thus, there is provided an ignitron firing control circuit capable of simple phase control by the setting of the tap 48. In the specific case illustrated, the phase control will be effective over a 60 degree ignitron conduction angle since phase II 'was selected as 120 degrees leading. If phase II were 60 degrees leading with respect to phase I, control of 120 degrees of the igniter firing cycle would be afforded.

Table I Component: Description 26 Transformer, 110 volt, 15 amp. R.M.S., approx. 1.6 k.v.a.

38 Transformer, 6 volt, 1 amp.

50, 48 Autotransfor-mer, Powerstat Type 10.

34 Silicon controlled rectifier, GE SCR35R, 16 amp., 200 volt.

46 Diode, 50 volt, 100 milliamps.

32 Resistor, 4 ohms, 1500 watts.

44 Resistor, 50 ohms, -1 watt.

The circuit shown in FIG. 1 has the advantage of simplicity in installation with a conventional three-phase power supply. In many applications, it may be desirable to provide phase control over a more extended position of the ignitron conduction angle. In such case, the cii cuit shown in FIG. 3 may advantageously be employed.

In FIG. 3 there is shown the ignitron serially coupled with a load circuit 18 across the secondary 20 or a power transformer 22. The primary winding 24 is coupled across source 25'. Firing current for the igniter electrode 16 of ignitron 10 is derived from transformer 38 coupled to the igniter electrode through resistor 32 and controlled rectifier 34 as set forth in connection with FIG. 1. Similarly, the primary winding 40 is supplied with power from the variable tap 48 through a resistor 50 coupled across source 25-.

The voltage applied to the gate electrode 36 over controlled rectifier 34 is, however, supplied from a transformer secondary 60 wound on transformer core 22 through phase shifter 62. The power derived from the phase shifter 62 is coupled across the gate-cathode circuit of rectifier 34 through the serially coupled combination of resistor 44 and rectifier 46. The operation is identical with that of the circuit shown in FIG. 1. The phase shifter 62 does, however, provide continuous phase control over the major portion of the ignitron current conduction angle.

This invention may be variously embodied and modified within the scope of the subjoined claims.

What is claimed is:

l. A control circuit comprising a first source of alternating voltage, a load, an ignitron having plate, cathode and igniter electrodes, said load being serially coupled with the plate and cathode electrodes of said ignitron across said first source, a second source of alternating voltage, a controlled rectifier having a gate electrode coupling said second source between said cathode and igniter electrodes of said ignitron, and means for applying a control voltage to the gate electrode of said rectifier to fire said rectifier at a predetermined point on the cycle of the wave form of said second source, said last named means comprising a diode, an alternating voltage coupled to said gate electrode by said diode, and means for controlling the time at which said control voltage reaches the firing voltage.

2. A control circuit in accordance with claim '1 in which said second source is in phase with said first source and in which said time control means includes means for shifting the phase of said control voltage with respect to the second source voltage.

3. A control circuit comprising a first source of alternating voltage, a load, an ignitron having plate, cathode, and iguiter electrodes, said load being serially coupled with the plate and cathode electrodes of said ignitron across said first source, a second source of alternating voltage leading in phase with respect to said first source, a controlled rectifier having a gate electrode coupling said second source between said cathode and igniter electrodes of said ignitron, means for deriving a variable amplitude voltage from said first source, and a diode, said diode coupling said variable amplitude voltage to said gate electrode of said con-trolled rectifier.

4. A control circuit in accordance with claim 3 in :which said means for deriving a variable amplitude voltage comprises an autotransformer having a variable tap thereon, said autotransforrner being coupled across said first source, the variable tap being coupled to said gate electrode through said diode.

References Cited in the file of this patent UNITED STATES PATENTS 2,110,700 Elder Mar. 3, 1938 2,147,472 Ulrey Feb. 14, 1939 2,182,633 Klemperer Dec. 5, 1939 

1. A CONTROL CIRCUIT COMPRISING A FIRST SOURCE OF ALTERNATING VOLTAGE, A LOAD, AN IGNITRON HAVING PLATE, CATHODE AND IGNITER ELECTRODES, SAID LOAD BEING SERIALY COUPLED WITH THE PLATE AND CATHODE ELECTRODES OF SAID IGNITRON ACROSS SAID FIRST SOURCE, A SECOND SOURCE OF ALTERNATING VOLTAGE, A CONTROLLED RECTIFIER HAVING A GATE ELECTRODE COUPLING SAID SECOND SOURCE BETWEEN SAID CATHODE AND IGNITER ELECTRODES OF SAID IGNITRON, AND MEANS FOR APPLYING A CONTROL VOLTAGE TO THE GATE ELECTRODE OF SAID RECTIFIER TO FIRE SAID RECTIFIER AT A PREDETERMINED POINT ON THE CYCLE OF THE 